Project D2: Durability of light alloys
D2.1: Automotive Mg alloys
•	Key researchers: A. Atrens, M.C. Zhao, Z. Liu
Summary
During 2007, Professor Andrej Atrens and Dr Ming-Chun Zhao carried out specific research at EMPA (Swiss Federal Laboratories for Materials Science and Technology, a Research Institute of the ETH Domain (Swiss Federal Institute for Technology)). This work provides a necessary foundation for the research on interrupted salt spray.
Recent work studied the influence of microstructure on the corrosion of as-cast ZE41. The microstructure of as-cast ZE41 consisted of the α-Mg matrix with the eutectic micro-constituent located along and/or adjacent to the boundaries of the α-Mg dendrites, showing a somewhat netlike distribution that was non-continuous. The corrosion rate of ZE41 in 1 N NaCl at room temperature was approximately double that of AZ91D and about thirteen times that of pure Mg. Conversely, if the corrosion area was considered, the corrosion of ZE41 was more homogeneous than that of as-cast AZ91D. Corrosion of ZE41 initiated in the α-Mg matrix adjacent to the eutectic micro-constituent, Figure D3, and was attributed to micro-galvanic corrosion of the α-Mg matrix coupled to the eutectic micro-constituent. The eutectic micro-constituent did not act as a corrosion barrier and did not stop the advance of the corrosion. As a consequence, α-Mg matrix corroded over the whole surface with little corrosion of the inter-connected eutectic micro-constituent.
				An improved method was also used to investigate the influence of crystallographic orientation on the corrosion of pure magnesium in 0.1 N HCl. The corrosion depth and orientation of surface features were mapped against crystallographic orientation (obtained by electron backscatter diffraction) for many off-principal magnesium crystals. The grains near (0 0 0 1) orientation are the most corrosion resistant. Most grains exhibited a striated structure of long and narrow hillocks with a unique direction.
Figure D3 - Corrosion of ZE41 initiated in the α-Mg matrix adjacent to the eutectic micro-constituent.
D2.2: Conversion coatings for Al/Mg alloys
•	Key researchers: A. Kumar, B.C. Muddle
Summary
Intense work has continued on the identification of appropriate sol-gel conversion coatings for high strength Al alloy AA2024. The coatings are being explored as candidates to replace toxic chromate based coatings. A deliberate focus is placed on the factors controlling interaction between coating and alloy substrate. As a result, the study has included the effect of surface treatment prior to coating (pre-treatments include grinding, polishing, chemical treatment and boiling).
The coatings are intended to serve as a means for superior corrosion resistance for such alloys. Furthermore, there is a deliberate emphasis examining the effect of surface microstructure on the resultant coating quality by means of the corrosion response on both a macro and localised (micro) scale. Consequently the past year has predominantly seen an emphasis on the collection of experimental data, including:
•	Synthesis of intermetallic particles that populate 2024 in order to study such compounds independently.
•	Electrochemical study of the effect of aqueous grinding on corrosion response of AA2024-T3.
•	Electrochemical study of the effect of surface pretreatment on Al-4Cu.
•	Initial testing via the micro-electrochemical capillary cell to ascertain corrosion response on the micro-scale.
D2.3: Design and development of corrosion resistant light alloys
•	Key researchers: A. Südholz, N. Birbilis, K. Meyer
Summary
Based upon thermodynamic and kinetic considerations, attempts are being made to explore/develop new alloy systems based on Mg and Al that exhibit superior corrosion resistance. Figure D2 shows some results from this effort. Figure D4(a) shows the resultant corrosion morphology that results upon Mg alloy AZ91E following 24 hours of immersion in 0.1M NaCl. Heavy localised corrosion associated with the (light) intermetallic particles is observed. Figure D4(b) however shows the same alloy with a quaternary alloying addition subject to the same exposure, revealing significantly less corrosion overall.
Figure D4 - Corrosion morphology following 24h immersion in 0.1M NaCl for (a) AZ91E, and (b) AZ91E with a trace alloying addition.
Studies were focused on the standard Mg-based alloy (AZ91) whilst exploring the effects of atypical alloying additions on thermodynamic and kinetic stability with regards to corrosion activity in neutral pH dilute chloride solutions. This included investigation of AZ91E with additions of Y, Ca, Ti, Li, B, Bi, Ce, Sc, Cr, Pb, Se, Sr.
The effect of binary additions of Ce, La, and Nd were also investigated upon pure Mg, over a range of alloying from 0 to 6wt%.
A protocol for imaging nanostructure of light alloys in air and in liquids has successfully been established via Atomic Force Microscopy. This is a major development in the ability to study the initial stages of corrosion and to correlate these to specific microstructural features. Atomic force microscopy allows not only high-resolution imaging (well beyond optical microscopy), but allows imaging to be carried out in situ in liquid (not possible with electron microscopy).
An example of this may be seen in Figure D5, whereby the features collected via TEM can be distinguished from those obtained via AFM (in which grain boundaries and precipitates can be distinguished).
Figure D5 - (a) Scanning TEM image of a grain boundary region of AA7075-T651; (b) AFM image of the same sample revealing grain structure and precipitates.